1. Field of the Invention
The present invention relates to a zoom lens and to an image pickup apparatus having the zoom lens.
2. Description of the Related Art
In recent years, zoom lenses having large aperture ratios, high zoom ratios, and high optical performance have been demanded for image pickup apparatuses, such as television cameras, silver-halide film camera, digital still cameras, and video cameras.
In image pickup apparatuses, such as broadcasting color television cameras, color separation optical systems and various filters are disposed at a front side (object side) of an image sensor. Accordingly, zoom lenses having long back focal distances have been demanded therefor.
A so-called positive-lead type four-unit zoom lens, in which a lens unit having a positive refractive power is disposed at the most object side, is known as the zoom lens having a large-aperture, a wide angle of view, a high zoom ratio, and a long back focal distance.
The four-unit zoom lens includes, in order from an object side to an image side, a first lens unit having a positive refractive power, which includes a focusing lens unit, a second lens unit (variator lens unit for varying magnification) having a negative refractive power, a third lens unit having a positive refractive power, which compensates for variation of an image plane due to magnification variation, and a fourth lens unit (image forming lens unit) having a positive refractive power.
U.S. Pat. No. 5,268,790 and No. 6,157,494 discuss a positive-lead type four-unit zoom lens capable of correcting chromatic aberration with a diffractive optical element located in the zoom lens to enhance optical performance.
U.S. Pat. No. 5,268,790 discusses a zoom lens having a zoom ratio of about 10 with a diffractive optical element located in a second or third lens unit.
U.S. Pat. No. 6,157,494 discusses a zoom lens having a zoom ratio of about 10 with a diffractive optical element located in a third lens unit.
The correction for chromatic aberration is facilitated by using a diffractive optical element in a part of the optical system of the zoom lens. Thus, zoom lenses having high zoom ratios and high optical performance can easily be configured.
However, in a case where the position and the optical power of a diffractive optical element to be located in an optical system of a zoom lens and the lens configuration of a lens unit including the diffractive optical element are inappropriately set, it is difficult to favorably correct chromatic aberration and to achieve high optical performance.
For example, in the case of the above-described positive-lead type four-unit zoom lens, it is important for increasing an aperture ratio and a zoom ratio to appropriately set the lens configuration of the third lens unit that is located closer to an image side than the second lens unit and that compensate for variation of an image plane due to magnification variation.
In a case where the lens configuration of the third lens unit is inappropriately set, even the zoom lens provided with a diffractive optical element neither reduces the variation of aberration caused by zooming, (for example, that of chromatic aberration, spherical aberration, halo, coma, and color difference due to spherical aberration) nor achieves a high zoom ratio and high optical performance.
Particularly, when the zoom ratio is enhanced, axial chromatic aberration at the telephoto end is increased. Consequently, it is difficult to correct residual aberration (secondary spectrum).